"Let's share the load" is a much less helpful way to coordinate cleaning the house than "You take the bedrooms, I'll do the kitchen" - unless there are 12 bedrooms. Vague plans can be useful tools to support joint action, but using vagueness effectively is a difficult computational problem. Participants in a joint planning study selected between specific and vague plans to coordinate action across a systematic range of problems. In Experiment 1, participants deployed vague plans selectively, recognizing situations where the certainty of a bad plan is outweighed by the flexibility of a vague plan according to a probabilistic model of joint reasoning. In Experiment 2, participants with greater exposure to such situations endorsed vague plans when providing generic testimony to future actors. Our results highlight an understudied but potentially powerful dimension of human joint planning: strategic use of vague construals.